1. Field of the Invention
The present invention broadly relates to real-time coherent optical filtering of imagery and, more particularly, is concerned with a method of spatial filtering which permits improved filtered image detection and spatial frequency bandwidth adjustment.
2. Description of the Prior Art
Recent psychophysical studies clearly indicate the presence of spatial frequency selective filters or channels within the visual system. Evidence from research suggests that these channels are quasi-independent, each varying from about one-to-two octaves in bandwidth.
More recently, studies show that the different spatial frequency channels are used to perform different visual tasks: low and mid frequencies are used for form perception and recognition while high frequencies are used for perception of fine detail. For example, Ginsburg (see "Specifying Relevant Spatial Information for Image Evaluation and Display Design: An Explanation of How We See Certain Objects," Proceedings of SID, 1980, vol. 21/3) demonstrated that a channel two octaves wide centered at 16 cpo (cycles per object) can be used to transmit all the information needed to recognize a human face.
The implication of these results is that the visual system is made up of many filters, each being responsible for capturing the spatial information needed to perform a particular visual task. If this view is correct then visual filtering processes for a particular task can be modelled by a spatial filtering system with similar characteristics.
However, until recently, real-time coherent optical filtering of imagery was accomplished solely by placing a spatial frequency mask in the Fourier transform plane of a coherent optical data processing system. The transfer function of the system was determined by the amplitude transmittance of the mask. Changing transfer functions required replacement and adjustment of numerous masks in the Fourier plane, a time-consuming problem. Such a system is inflexible if more than one special type of filtering is required.
Now, Indebetouw (see "Turnable Spatial Filtering with a Fabry-Perot Etalon," Applied Optics, Mar. 1, 1981, vol. 19, No. 5, pp. 761-764) has shown that flexible bandpass capability could be achieved by placing an angular filter in the image plane of the system. An angular filter is a device that is capable of selecting a set of plane waves with arbitrarily chosen direction cosines. Since direction cosines are proportional to the input spatial frequencies, the angular spectrum is a scaled version of the Fourier transform. Thus, the angular filter can select spatial frequency bands similar to a mask in the Fourier plane.
Indebetouw uses a Fabry-Perot etalon or interferometer as his angular filter. The center frequency and bandwidth of the filter can be easily chosen by varying the finesse (number of internal reflections within the mirror pair) and spacing of the mirror pair. The center frequency of the bandpass filter can be adjusted by small (less than 0.05 mm) changes in the mirror spacing of the interferometer, while the bandwidth of the filter can be adjusted by larger (greater than 0.25 mm) changes in the mirror spacing. The farther apart the overall spacing of the mirrors, the narrower the bandwidth.
While the Indebetouw system is a definite step forward in the field of real-time coherent optical filtering of imagery, certain improvements are needed in order to enhance its capabilities as a research tool for studying visual processes and as a laboratory tool for analysis of pictorial images. As mentioned above at the beginning, a model of visual processing has been developed which views the visual system as a filtering system. As such, it has been shown that certain bandwidths of spatial frequency information in images are used primarily by the visual system for detection and identification of targets. If these bandwidths can be properly enhanced with real-time spatial filtering, then the provision of an optical processing system with filtering characteristics more closely modelling human vision may be possible.